As people age, their eyes lose their ability to accommodate, that is to change the distance at which their eyes focus and, in particular, to focus on near objects. The eye accommodates by changing the shape of its lens, to produce a stronger or weaker lens to image nearer or farther objects, respectively. As the eye ages, the maximum focusing power that its lens can provide reduces. This loss of accommodation is known as presbyopia, and typically becomes noticeable in people's 30s, 40s or 50s. Presbyopia is traditionally compensated for by prescribing spectacles with magnifying lenses, that is lenses having a positive power, which supplement the focusing power of the eye to enable focusing on near objects.
Other common eye defects are myopia (short sight, wherein an object in the far distance is focused in front of the retina) and astigmatism (wherein the eye focuses light differently in two orthogonal planes). Myopia is compensated for by using a lens having negative optical power, i.e. a concave, defocusing lens. Astigmatism is compensated for by using a lens having different optical powers in the two orthogonal planes, equivalent to a spherical lens plus a cylindrical lens.
Where an eye has myopia and/or astigmatism, as well as presbyopia, the presbyopia is typically treated by providing a lens having different transverse regions with different powers. For example, a traditional bifocal spectacle lens, for a person with both myopia and presbyopia, has a mid/upper region for distance vision, and a lower region for near vision. The distance vision region has negative optical power. The optical power of the near vision region is positive, relative to the distance vision region. So, for example, a myope requiring a −3 diopter (D) lens portion for distance vision may require only a −2D lens portion for near work; the near vision region of the lens then has an optical power of +1 D, relative to the distance vision region. The lens is then said to have an “add power” of +1 D.
Like a myope, an emmetrope (a person with normal vision) will also lose the ability to accommodate as they get older. For near work, they will need a corrective lens having just the add power, which will actually be the power of the lens (e.g. the base power of the lens is 0 D and the add power is +2D then it is a +2D lens).
A hyperope (a person with long-sight) will also lose the ability to accommodate as they get older. They will already need a positive power of lens to correct their hyperopia, and will need a further add power for near work. For example, a +3D hyperope may require +1 D add power, resulting in a power of +4 D in the near vision region.
Astigmatism further complicates the situation. A myope with vertical astigmatism may require, for example, a −2D lens to compensate for the myopia and an additional −2D in the 0 degrees-180 degrees plane (but not in the 90 degrees-270 degrees plane). As the astigmatic myope ages, there may be an add power requirement of, say, +1 D for near work, resulting in a lens with a near-vision region that is −3D in the 0 degrees-180 degrees plane and −1 D in the 90 degrees-270 degrees plane.
When wearing spectacles, a person can look through different regions of the lens by moving their eyes, looking through the mid/upper region for distance vision and looking down through the lower region for near vision. When wearing contact lenses, however, that is not possible, as the contact lens moves with the eye. One approach to compensating for presbyopia is the so-called monovision approach, in which a lens for distance vision is prescribed for one eye and a lens for near vision is prescribed for the other eye. However, a preferred approach is to provide one or more add-power regions on the contact lens. A common approach is to provide a contact lens with an optical zone (i.e. the zone through which light passes into the eye) having a central region proving near vision, surrounded by an annulus providing distance vision (N-type lens) or a central region providing distance vision, surrounded by an annulus providing near vision (D-type lens). A transition or blending region may be provided between the near region and the distance region. There may be one or more additional annulus, with each successive annulus being a near region or a distance region, alternately. Again, blending regions may be provided between each annulus.
Contact lenses having at least one near-vision region and at least one distance-vision region are referred to as multifocal contact lenses.
One of the front surface or back surface of the contact lens will be structured to provide the distance-vision correction, and the other surface will be structured to provide the near-vision correction. For example, the front of the contact lens may be a spherical surface having a radius of curvature appropriate for correction of the distance vision, and the back of the contact lens may have a central region having a radius of curvature that, in conjunction with the radius of curvature of the front region, provides the distance-vision correction, and an annular region having a radius of curvature that, in conjunction with the radius of curvature of the front surface, provides the near-vision correction. Alternatively, the necessary curvatures for both the distance-vision correction and the near-vision correction may be provided on the front surface of the contact lens.
Silicone hydrogel contact lenses have particularly desirable material properties, including high permeability to oxygen. The lenses are produced by cast moulding monomers to produce a dry lens, which is then hydrated. The hydration alters the dimensions of the lens, meaning that the power of the hydrated lens is different from that of the dry lens (and hence different from the apparent power of the mould used to make the lens). Furthermore, there is some variability in the hydration process, resulting in a variability in the power of the hydrated lens.
It is therefore desirable to measure the power of a hydrated contact lens, which can be either a hydrogel contact lens or a silicone hydrogel contact lens, to check that it has the correct power (within acceptable manufacturing tolerances).
It is straightforward to measure the power of a simple contact lens (i.e. a contact lens having only a single, spherical power), for example using a focimeter (or lensometer). It is also relatively straightforward to measure the power of each region of a multifocal contact lens. It can be done, for example, by tilting the lens or by using one or more masks to ensure that light can pass only through the region to be measured. For example, for a D-type lens with a central distance-vision region and one annular near-vision region, the distance power can be measured using a focimeter and a mask that allows light to pass only through the central region of the lens, and the near power can be measured using a focimeter and a mask that allows light to pass only through the near-vision annulus. However, power measurements using a focimeter can be affected by lens tilt, incorrect lens centering on the focimeter stage or the stop, poor transmission of light and the lens design profile. Additionally, as just described, when using a focimeter to measure the ‘add power’, two measurements of power are required, a measurement using a central aperture and a measurement in the annular region of the lens outside the central zone. However, in some contact lenses the power profile is changing with radial distance and is non-symmetric. That can result in the best measurement of power actually being the best focus point rather than the average power of the annular zone. Additionally, tilting the lens or changing stops requires the position of the lens to be changed between measurements, resulting in inconsistent lens placement.
A particular problem arises with multifocal contact lenses that are structured also to compensate for astigmatism. Such lenses are referred to as multifocal toric lenses, as their surfaces have different curvatures in two orthogonal directions, like the surface of a torus. It is relatively difficult to provide masks that reliably mask the correct portions of the lens for a focimeter measurement. Without masks, the complex structure of the lens—with multiple zones and lack of circular symmetry—typically results in there being no clear focal point that the focimeter can measure.
The present disclosure addresses these and other needs.